It's more productive but less profitable. The potential "advertisers" would only advertise with this network if they could purchase computing time through it at a lower rate than the spot price on Amazon EC2 -- otherwise they'd get their computation done faster and cheaper going to EC2. At the current spot prices EC2 charges, this network could not pay out within even two orders of magnitude of the CPMs even low-quality sites earn from ads now. The processing power of JavaScript time slices on commodity laptops is just too far apart from running compiled code on even the slowest EC2 instance size. That means it's unlikely for a significant publisher base to develop to sell the ads onto.

If you're interested in working on it, there's at least one or two startups trying it at any given time you could try to work with. http://crowdprocess.com/ is relatively new. Many have had the same idea and eventually closed down.